1. Field of the Invention
The present invention generally relates to a bandgap voltage reference circuit, more particularly, to a fast start-up low-voltage bandgap voltage reference circuit.
2. Description of the Prior Art
In general, reference voltage can be generated by voltage-dividing of resistors or by the self-bias of a transistor. However, such reference voltage is not independent of working voltage and temperature, as well as the variation in the manufacturing. In order to solve the problems, a bandgap voltage reference circuit is provided.
The principle of the bandgap voltage reference circuit is to implement components having characteristics of positive temperature coefficient and negative temperature coefficient respectively. And then add the voltages or the currents of these components in a predetermined proper proportion to generate a value independent of temperature, and such value can be output as a reference.
FIG. 1 is a diagram showing the bandgap voltage reference circuit in such kind. As shown, the transistors M1, M2, Q1, Q2, the resistor R1 and the amplifier OP1 form a self-bias circuit generating a current in positive proportion to                     V        T            ⁢      ln      ⁢                           ⁢      N        R1    ,wherein N is the Emitter Area Ratio of the transistors Q1 and Q2. Therefore, the bandgap voltage Vbg is:                     Vbg        =                              V            BE2                    +                                    R              2                        ⁢                                                            V                  T                                ⁢                ln                ⁢                                                                   ⁢                N                            R1                                                          (        1        )            
wherein VBE2 has a negative temperature coefficient −2.2 mV/° C., and VT has a positive temperature coefficient +0.85mV/° C. Aspect Ratio of M1, M2 and M3 are all equal, the Formula (1) can be rewritten as:             V      BG        ⁡          (      T      )        =            (                        V          BE0                -                  2.2          ×                                    10                              -                3                                      ·            Δ                    ⁢                                           ⁢          T                    )        +                                        (                                          V                T0                            +                              0.085                ×                                                      10                                          -                      3                                                        ·                  Δ                                ⁢                                                                   ⁢                T                                      )                    ⁢          ln          ⁢                                           ⁢          N                R1            ·      R2      
wherein ΔT=T−300 ° K (i.e. the difference of working temperature and the room temperature), VBE0 is VBE under room temperature and the value is around 0.6V, VT0 is VT under room temperature and the value is around 0.026V. In order to make the temperature coefficient of VBG equal to “0”, make                     ∂                  V          BG                            ∂        T              =    0    ,            then      ⁢                           -              2.2        ×                  10                      -            3                          ⁢        T            +                                                                  (                                  0.085                  ×                                      10                                          -                      3                                                        ⁢                  T                                )                            ·              ln                        ⁢                                                   ⁢            N                    R1                ·        R2              =    0.  
So,                               ln          ⁢                                           ⁢          N                R1            ·      R2        =    25.88    ,                                          V            T0                    ⁢          ln          ⁢                                           ⁢          N                R1            ·      R2        =                  25.88        ×        0.026            =      0.67        ,then make ΔT=0, and the Formula (1) will become:       V    BG    =                    V        BE0            +                                                  V              T0                        ⁢            ln            ⁢                                                   ⁢            N                    R1                ·        R2              =          0.6      +      0.67      +      1.27      
In general, VBG is around 1.27V, and the value varies depending on different manufacturing processes (for example, VBE0 may vary between 0.5V˜0.7V). Even the bandgap voltage Vbg independent of temperature can be obtained, however, it should be around 1.2V to offset the positive/negative temperature coefficient, which means this circuit will not work when the working voltage VCC is lower than 1.2 V.
FIG. 2 shows a low-voltage bandgap voltage reference circuit pretty common in prior art, in which the circuit will work under low VCC. As shown in FIG. 2, the resistor R2 is connected parallel to the resistors R3 and R4 having voltages Va and Vb respectively, which is a modification of the circuit shown in FIG. 1 in which the resistor R2 is connected serial to the voltage Vb. Assuming R3=R4 and the transistors M1, M2, Q1, Q2, the resistor R1 and the amplifier OP1 form a self-bias circuit. When the self-bias circuit is steady, the corresponding currents will be:                               I          R3                =                                            V              a                        R3                    =                                    V              BE1                        R3                                              (        2        )                                          I          Q1                =                                            V              T                        ⁢            ln            ⁢                                                   ⁢            N                    R1                                    (        3        )                                          I          M1                =                              I            M3                    =                                                    I                R3                            +                              I                Q1                                      =                                                            V                  BE1                                R3                            +                                                                    V                    T                                    ⁢                  ln                  ⁢                                                                           ⁢                  N                                R1                                                                        (        4        )            
Therefore, changing the proportion between the R1 and R3 will generate a current independent of temperature. With R5, the current can be transformed to the bandgap voltage Vbg as follows,                     Vbg        =                  R5          ⁡                      (                                                            V                  ⁢                                                                           ⁢                  BE1                                R3                            +                                                V                  ⁢                                                                           ⁢                  T                  ⁢                                                                           ⁢                  ln                  ⁢                                                                           ⁢                  N                                R1                                      )                                              (        5        )            
Since the circuit in FIG. 2 is achieved by the addition of currents (IR3+IQ1), it will not be limited by the condition that the working voltage should be around 1.2V (as the prior art illustrated in FIG. 1) and will work below 1V. However, when starting, the currents on the transistors Q1 and Q2 are much lower than that on the resistors R3 and R4, and also R3=R4, so the voltage Va is almost equivalent to Vb. In such circumstances, the amplifier OP1 will not pull up the self-bias voltage to a steady stage. Therefore, when starting, the self-bias circuit needs to be set up to a steady stage with an external reset signal. For example, As shown in FIG. 2, the starting unit 21 provides a reset signal to turn on an auxiliary transistor Mx when the self-bias circuit is not in the steady stage. And then the starting unit 21 has to monitor the current Ix on the transistor M0 to turn off the auxiliary transistor Mx when the current Ix reaches to a threshold value (i.e. when the self-bias circuit reaches the steady stage). In one embodiment, the starting unit 21 comprises a power-on reset circuit.